A power steering system of an automotive vehicle comprises a pump for providing hydraulic fluid under pressure. A typical power steering pump comprises a rotor having retractable vanes and rotating within a cam chamber. During operation, hydraulic fluid is drawn into the cam chamber from a fluid suction passage and pumped out under pressure to a fluid discharge port. A rotor is driven by the engine through a belt and pulley. As the speed of the engine increases, the volume of fluid pumped by the rotor also increases, and exceeds the volume required by the power steering system for optimum operation. The output from the pump is maintained at an optimum value by recycling a portion of the pumped fluid through a bypass in the pump housing, so that pumped fluid is diverted from the outlet and returned to the suction passage. At low engine speeds, the bypass is closed so that the entire volume of pumped fluid is outputted from the pump. However, at higher engine speeds, the bypass is open for recycling as much as 90% of the pumped fluid.
U.S. Pat. No. 5,887,612, issued Bleitz et al. in 1999, shows a mechanical valve for opening and closing a fluid bypass port to regulate the output from the pump. For this purpose, the housing defines a bore that communicates with a fluid discharge port from the pumping chamber and with the fluid bypass port. The outlet from the pump is located at one end of the bore and comprises a constricted passage to limit fluid output to a desired volume. Within the bore, a flow control valve slides to open and close the bypass port. The valve is biased in the closed position by a spring. During operation, particularly at higher engine speeds, the increased pressure of fluid from the fluid bypass port acts upon the valve to contract the spring and open the bypass port, thereby diverting excess fluid from the outlet and recycling fluid through the bypass port.
Because of the restrictive outlet, the pump maintains a relatively constant output volume. Since the output is restricted, the excess fluid tends to push the spring-biased valve into the fully open position at higher engine speeds. This effectively results in energy being wasted from turning the pump to provide fluid that is not required. It is known to provide a variable volume power steering pump wherein the size of the cam chamber is varied. During operation, the volume of pumped fluid is regulated by adjusting the cam chamber, and does not required fluid to be recycled through a bypass port. However, mechanisms for controlling the size of the cam chamber are complicated and require additional in-pump components and controls.
Therefore, a need exists for a power steering pump having a variable output, in which, a portion of the pumped fluid is recycled through a bypass within the pump, and further in which the portion through the bypass is controlled to obtain a desired output from the pump.